1. Field Of The Invention
This invention relates to biphenyltetracarboxylic dianhydrides and a process for their production.
2. Description Of The Prior Art
It has been known that 2,3,2',3',- and 3,4,3',4'-biphenyltetracarboxylic dianhydrides are useful, for example, as starting material for producing of polyimides. See, e.g., U.S. Pats. Nos. 3,179,614 and 3,179,630 to 3,179,633.
However, no method is yet known which is commercially acceptable for the mass-production of these dianhydrides.
Previously (Japanese Patent Applications 46-87579, 46-91136, 47-32554, 47-49143) we studied a method of producing biphenyltetracarboxylic dianhydrides on an industrial scale. As a result, we found that when dimethyl ortho-phthalate or ortho-xylene is reacted in the presence of an organic palladium salt and a certain additive under oxygen pressure, a dimerized product thereof (a mixture of three isomers), which is an oxidative coupling reaction product, can be produced at high yield, the this dimer can be easily converted to an isomeric mixture of biphenyltetracarboxylic acids by a procedure such as hydrolysis or oxidation, and that when the resulting mixture is heated biphenyltetracarboxylic dianhydrides can be produced commercially.
However, the isomeric mixture of biphenyltetracarboxylic dianhydrides so obtained contains 2,3,3',4'- (10.about.80%) and 3,4,3',4'-(90.about.17%) isomers as the main products and unreacted biphenyltetracarboxylic acid isomers as impurities. This isomeric mixture is extremely difficult to separate and purify at good yields, even by recrystallization or distillation. Although the mixture also contains a 2,3,2',3'-isomer (0.about.3%), since the amount of this isomer is very small at the time of producing the dimerized product, it is also present in only small amounts (0.about.3%) in the isomeric mixture of biphenyltetracarboxylic dianhydrides.
This led us to an extensive study on methods of separating the desired isomers, especially the 2,3,3',4'- and 3,4,3',4'-isomers, in pure form from the isomeric mixture of biphenyltetracarboxylic dianhydrides. Consequently, we found that diphenyltetracarboxylic dianhydrides vary greatly in their solubility in aliphatic acid anhydrides or ketones according to the type of isomer, and the isomers can be easily separated according to this difference in solubility. We further found that when the isomers are recrystallized from the aliphatic acid anhydrides, the unreacted biphenyltetracarboxylic acids present in the isomers can be converted to biphenyltetracarboxylic dianhydrides and small amounts of other impurities can be separated and removed, whereby the desired isomers of biphenyltetracarboxylic dianhydride can be obtained in pure form.
As previously stated (Japanese Patent Application 46-87579), we found that when dimethyl ortho-phthalate is reacted in the presence of an organic palladium salt and a certain additive under oxygen pressure, tetramethyl biphenyltetracarboxylate, its dimer, is produced at high yield by oxidative coupling. This tetramethyl biphenyltetracarboxylate comprises 2,3,2',3'-, 2,3,3',4'- and 3,4,3',4'-isomers. Although differing somewhat according to the conditions of producing the tetramethyl biphenyltetracarboxylates, the proportions of the isomers are about 0.about.3% for the 2,3,2',3'-isomer, about 54-80% for the 2,3,3',4'-isomer, and about 43.about.17% for the 3,4,3',4'-isomer. Since the amount of the 2,3,2',3'-isomer is extremely small, it can be separated to some extent by distillation, and can be ignored for many practical purposes. However, we found that it is very difficult to economically separate and purify the 2,3,3',4'- and 3,4,3',4'-isomers.
These isomers are insoluble in water and they cannot be separated by mere recrystallization from an organic solvent such as methanol, ethanol, tetrahydrofuran, methyl acetate, carbon tetrachloride or methyl cellosolve. Furthermore, because of their high boiling point, it is not practical to separate and purify them by distillation. When the isomeric mixture is subjected to fractional crystallization using an organic solvent such as methanol, butanol, ether, tetrahydrofuran, toluene, methyl cellosolve, nitrobenzene, chloroform, carbon tetrachloride, acetone, methyl acetate or ethanol-ether, if the ratio of the 2,3,3',4'- and 3,4,3',4'-tetramethyl biphenyltetracarboxylates is at least 1:2, the constituent which is present in excess is first precipitated, but both isomers in a ratio of about 1:1 still remain in the solution. They cannot be separated economically and quantitatively.